In general, the light emitted by a semiconductor diode comes from the recombination of electrons and holes as a result of the flow of charge through a forward biased P-N junction. At sufficiently high current density the junction exhibits optical gain. To achieve laser operation, the optical gain provided by the diode in the shape of a long narrow stripe must be combined with optical feedback. Usually, feedback is achieved by cleaving a pair of oppositely disposed end facets of the semiconductor crystal containing the P-N junction. The cleaved end facets are oriented perpendicular to the striped P-N junction which also serves as a waveguide. Reflections back into the waveguide from the cleaved end facets provide the regeneration required for laser operation. The laser output radiation is transmitted through one or both of the cleaved facets.
The cleaved crystal end facets in combination with the waveguide along the junction define a conventional Fabry-Perot waveguide cavity resonator. With respect to the waveguide cavity resonator, "longitudinal" designates the direction of propagation along the waveguide and "lateral" and "transverse" designate the two directions perpendicular to the direction of the waveguide axis. An important aspect of semiconductor laser diode design is to reduce the number of lateral and transverse cavity modes so that the laser output is spatially coherent and capable of diffraction-limited collimation and to reduce the number of longitudinal cavity modes so that the output radiation is as monochromatic as possible. When there is only one longitudinal cavity mode the output is substantially a single frequency, that is, the entire output is within a very narrow frequency band.
The most common type of laser diode structure is known as the double heterostructure. The double heterostructure may be utilized to produce a collimated spatially coherent output beam. Typically the double heterostructure is formed from a ternary or quaternary material such as AlGaAs or GaInAsP. The double heterostructure laser comprises a narrow bandgap, optically active thin layer (0.1-0.2 micron thick) which is sandwiched between a pair of thicker, wider bandgap cladding layers. The index of refraction and the band structure change abruptly at the interfaces between the active layer and the cladding layers. The cladding layers serve to confine charge carriers to the active layer and, because they have a lower index of refraction, provide fundamental mode waveguiding in the transverse direction for radiation propagating in the plane of the active layer. Fundamental mode waveguiding in the lateral direction may be achieved by defining a relatively narrow stripe-shaped active region, which stripe-shaped active region is also bound laterally by wide bandgap material as in the cladding layers. This type of structure is commonly referred to as "index-guided". The width of the active layer stripe is about 1.5 microns in order to achieve fundamental lateral mode guiding. This dimension depends on the thickness. An example of a laser diode having a narrow stripe-shaped index-guided active region is the double channel buried heterostructure. When the waveguide has sufficiently small dimensions in the transverse and lateral directions, only the fundamental transverse mode and the fundamental lateral mode are supported and can oscillate so that a spatially coherent Output results.
In the typical double-heterostructure laser, the crystal end facets are cleaved to provide reflecting surfaces. As indicated above, the cleaved end facets define a Fabry-Perot cavity resonator for supplying feedback. Because of its length (typically 250 microns) the Fabry-Perot waveguide cavity can in principle support a large number of longitudinal modes. However, only those longitudinal modes within the wavelength band in which the semiconductor material has gain actually oscillate. Thus, a typical double heterostructure Fabry-Perot type laser oscillates in a significant but limited number of longitudinal modes.
In many applications only one longitudinal mode can be tolerated. One way to reduce the number of oscillating longitudinal modes is to provide some periodic structure that couples to the radiation field within or near and parallel to the active region. For example, one can construct a fundamental transverse mode and fundamental lateral mode waveguide laser diode with a periodic index of refraction perturbation grating incorporated adjacent to the waveguide. The grating with carefully chosen periodicity forms a distributed reflector that turns the waveguide itself into a wavelength sensitive resonator which supplies feedback only over a very narrow wavelength band. The feedback is provided through periodic in-phase reflection from the grating elements. The reflections are in phase at only one wavelength. At other wavelengths the reflections cancel. This technique is known as distributed feedback. Deleterious broad bandwidth Fabry-Perot type feedback is suppressed by putting anti-reflection coatings on the crystal end facets of the laser. Since effective feedback is present for only a narrow wavelength band, and the possible longitudinal modes are spaced in wavelength as in the Fabry-Perot, the laser oscillates in only a single longitudinal mode and the laser output is substantially monochromatic. Another approach to achieving single frequency output from a semiconductor diode laser involves use of a narrow band-pass plane wave reflecting filter that is formed external to the semiconductor laser. Such an approach is described in N. K. Dutta, et al., "Single Longitudinal Mode Operation of a Semiconductor Laser Using a Metal Film Filter", Journal of Ouantum Electronics, Vol. QE-21, No. 6, June 1985, pp 559.
The most common single frequency output laser diode is the distributed feedback laser described above. However, the distributed feedback laser exhibits chirping during current modulation and has a broad linewidth much greater than 1 Mhz. Thus, the distributed feedback laser is suitable only for use in incoherent, long fiber, high speed systems. In such systems, the single frequency output is not subject to partition noise of the type exhibited by multi-frequency Fabry-Perot type lasers. However, it cannot be used for coherent systems.
Laser diodes utilizing an external element for feedback generally have very narrow band output and exhibit minimal chirping because most of the energy stored in the cavity is outside the diode. Therefore, they can be used in coherent systems.
Accordingly, it is an object of the present invention to provide an alternative mechanism for achieving narrow band single frequency output with minimal or no chirp during current modulation from a semiconductor laser diode and more particularly, it is an object of the present invention to utilize a conventional Fabry-Perot type laser diode in a manner so as to achieve a single frequency output. This avoids development of a new laser diode structure.